Conventional image forming apparatuses and the like employ a belt device for conveying paper or a developer image and for performing intermediate transfer of a developer image. This type of belt device employs a plurality of rollers and an endless belt looped around the rollers. The endless belt may become skewed and shift in position in the widthwise direction of the belt due to forces acting in the widthwise direction. Techniques for preventing skewing and widthwise positional shift in the belt have been proposed. For example, a technique disclosed in Japanese Patent Application Publication No. 2003-255642 provides a guide rib on one side of the belt and a groove formed in a roller for engaging with the guide rib to prevent skew toward the other side of the belt. This technology also urges both ends of a rotational shaft in a roller with springs in order to apply tension to the belt along the circumference thereof. The urging force of the spring on the side of the roller near the groove is set greater than the urging force of the spring on the opposite side of the roller in order to produce a greater tension along the circumference of the belt on the side having the guide rib than the opposite side of the belt. This tension differential in the belt produces a force component acting in a direction from the side having the guide rib toward the side not having the guide rib. Consequently, the guide rib and groove structure restricts movement of the belt from the guide rib side toward the side not having the guide rib and also prevents skewing and widthwise positional shift of the belt by pulling the belt from the side having the guide rib toward the side not having the guide rib.